1. Field of the Invention
This invention relates to a process for production of microcapsules. More particularly it is concerned with a process for production of fine microcapsules in which a polyvalent isocyanate, a polyvalent isothiocyanate or a prepolymer thereof is incorporated into a hydrophobic oily liquid to be encapsulated and emulsified in an aqueous solution of a water-soluble polymer whereby at the stage of forming capsules by polymerizing melamine and formaldehyde, precondensates thereof or urea and formaldehyde, the stability of the emulsion is increased, and flocculation and flotation of the oily liquid are prevented.
2. Description of the Prior Art
Methods of forming capsules with a shell of a urea-formaldehyde resin are classified into a method in which encapsulation is effected after forming a precondensate of urea and formaldehyde which is soluble in water and an oily liquid, and a method in which capsules are formed simultaneously with the polycondensation of urea and formaldehyde. The former method is described in U.S. Pat. Nos. 3,016,308, 3,516,846 and 3,993,831, Japanese Patent Publication Nos. 12380/62, 12518/63, 30282/71, 29483/70, 14379/69, 18671/77, Japanese Patent Application (OPI) No. 7313/71 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"), etc. The latter method is described in U.S. Pat. Nos. 4,001,140 and 4,089,802. When compared with the method in which the precondensate is used, the method in which urea and formaldehyde are used and the capsules are formed simultaneously with the polycondensation of the urea and formaldehyde is preferred because the production steps are simpler and a highly concentrated solution can be prepared. This method, however, suffers from the disadvantages that microcapsules cannot be formed unless a water-soluble polymer having a sufficiently high emulsion stabilizing capability is employed and for this purpose only limited polymers are suitable. Furthermore, the temperature of the encapsulation reaction must be increased as compared with the method in which the precondensate is formed, and coalescence of the oily liquid and flotation of the oily particle on the surface as explained later take place.
The formation of microcapsules of an oily liquid together with the polymerization of urea and formaldehyde in the presence of a water-soluble polymer is disclosed in U.S. Pat. Nos. 4,001,140 and 4,089,802. This method comprises: (1) dissolving urea and resorcinol in an aqueous solution of a water-soluble polymer and the resulting solution is adjusted to near 3.5 in pH (vehicle preparation), (2) emulsifying a hydrophobic oily liquid to be encapsulated in the aqueous solution prepared at step (1), (3) adding formaldehyde to the emulsion prepared at step (2) (chemical addition), and (4) encapsulation--the resulting mixture is heated with stirring to cause the polycondensation of urea and formaldehyde whereby capsule walls are formed.
A method of forming capsules having a shell of a melamine-formaldehyde resin is disclosed in U.S. Patent No. 4,100,103 and Japanese Patent Application (OPI) No. 49984/79. This method relies upon a copolymer of maleic anhydride and an ethylene based monomer or an aqueous solution of polyacrylic acid as a water-soluble polymer and it involves (1) emulsifying a hydrophobic oily liquid, (2) mixing the resulting emulsion with an aqueous solution of melamine and formaldehyde or a precondensate of melamine and formaldehyde, and (3) adjusting the pH value of the resulting mixture and raising the temperature thereof to effect the encapsulation.
The water-soluble polymers used in the above methods have four roles. They function as an accelerator form emulsifying the oily liquid to be encapsulated, as a stabilizer of the emulsion, as an accelerator for forming a capsule wall, and as a dispersant of the capsule. Among these roles, their ability to stabilize the emulsion, in particular, is insufficient.
For example, when an isobutylene-maleic anhydride copolymer, polyacrylic acid or a vinyl acetate-maleic anhydride copolymer is used, the hydrophobic oily liquid is not completely emulsified in the emulsification step and a film of the hydrophobic oily liquid covers the surface of the emulsion. In the chemical addition step and encapsulation step, the oily liquid particles in the emulsion flocculate or combine, accelerating the formation of the film of the oily liquid and forming large particles. Since the film of the oily liquid remains unencapsulated, for example, in coating a capsule solution on paper for the production of a pressure-sensitive recording paper, the film repels the coating solution and a uniformly coated product is not obtained. Furthermore, large particles of the oily liquid are encapsulated as they are. Therefore, when such capsules are used in the production of a pressure-sensitive paper, they rupture upon application of the slightest force, leading to serious problems such as contamination of paper.
In addition, even when a water-soluble polymer having relatively good capability to stabilize the emulsion is employed, if the temperature of the system is raised or the pH value is reduced in order to accelerate encapsulation, the stability of the emulsion is reduced, leading to the flotation of the oil and the formation of the large particles.
Moreover, a reduction in the amount of the water-soluble polymer, which is effective to lower the viscosity and reduce costs, is limited because it deteriorates the stability of the emulsion.
Another disadvantage is that the viscosity of the capsule solution prepared is high. This is mainly due to the viscosity of the polymer being used. However, when the molecular weight of the polymer is decreased or the amount of the polymer added is decreased in order to reduce the viscosity of the capsule solution, the stability of the emulsion is further reduced, leading to formation of the film of the oily liquid and of large particles.
This phenomenon is accelerated by lengthening the storage period of the emulsion and by raising the encapsulation temperature.
As already disclosed in Japanese Patent Application No. 15675/78, the use of gum arabic in encapsulation creates excellent effects, for example, it reduces the viscosity of the system, improves washability, and accelerates encapsulation. However, it has the defect that large capsules are easily formed due to the unsuitable stability of the emulsion. The phenomenon is accelerated by lengthening the period of the chemical addition step or by raising the temperatuure at encapsulation step.